Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, and power metal oxide semiconductor field effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, charged-coupled devices (CCDs), solar cells, and digital micro-mirror devices (DMDs).
Semiconductor devices perform a wide range of functions such as signal processing, high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, and creating visual projections for television displays. Semiconductor devices are found in the fields of entertainment, communications, power conversion, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment.
Semiconductor devices exploit the electrical properties of semiconductor materials. The atomic structure of semiconductor material allows its electrical conductivity to be manipulated by the application of an electric field or base current or through the process of doping. Doping introduces impurities into the semiconductor material to manipulate and control the conductivity of the semiconductor device.
A semiconductor device contains active and passive electrical structures. Active structures, including bipolar and field effect transistors, control the flow of electrical current. By varying levels of doping and application of an electric field or base current, the transistor either promotes or restricts the flow of electrical current. Passive structures, including resistors, capacitors, and inductors, create a relationship between voltage and current necessary to perform a variety of electrical functions. The passive and active structures are electrically connected to form circuits, which enable the semiconductor device to perform high-speed calculations and other useful functions.
Semiconductor devices are generally manufactured using two complex manufacturing processes, i.e., front-end manufacturing, and back-end manufacturing, each involving potentially hundreds of steps. Front-end manufacturing involves the formation of a plurality of die on the surface of a semiconductor wafer. Each die is typically identical and contains circuits formed by electrically connecting active and passive components. Back-end manufacturing involves singulating individual die from the finished wafer and packaging the die to provide structural support and environmental isolation.
One goal of semiconductor manufacturing is to produce smaller semiconductor devices. Smaller devices typically consume less power, operate with a lower voltage, have higher performance, and can be produced more efficiently. In addition, smaller semiconductor devices have a smaller footprint, which is desirable for smaller end products. A smaller die size may be achieved by improvements in the front-end process resulting in die with smaller, higher density active and passive components. Back-end processes may result in semiconductor device packages with a smaller footprint by improvements in electrical interconnection and packaging materials.
Most modern electronic equipment requires a power supply to provide a direct current (DC) operating potential to the electronic components contained therein. Common types of electronic equipment which use power supplies include personal computers, energy systems, telecommunication systems, audio-video equipment, consumer electronics, automotive components, and other devices which utilize integrated circuits, semiconductor chips, or otherwise require DC operating potential. Most, if not all, semiconductor components require a DC operating potential. However, many sources of electric power are alternating current (AC), or high voltage DC, which must be converted to a lower voltage DC for the electronic equipment.
In one common arrangement, the AC/DC power supply receives an AC input voltage, e.g., between 110 and 240 VAC, and converts the AC input voltage to the requisite DC operating voltage. The AC voltage is routed through a full-wave rectifier bridge and filtered to produce a high voltage DC signal. The high voltage DC signal is processed through a pulse width modulated (PWM) controller and transformer assembly to generate the low voltage, regulated DC output voltage, which is used as the operating potential for the semiconductor components and other devices requiring low voltage DC supply in the electronic equipment. The low voltage DC signal is typically in the range of 5 to 12 VDC. In other cases, a DC/DC power supply receives a high voltage DC signal and provides the low voltage DC signal necessary for the electronic equipment.
MOSFETs are commonly used in electronic circuits, such as communication systems and power supplies. Power MOSFETs are particularly useful when used as electric switches to enable and disable the conduction of relatively large currents. The on/off state of the power MOSFET is controlled by applying and removing a triggering signal at the gate electrode. When turned on, the electric current in the MOSFET flows between the drain and source. When turned off, the electric current is blocked by the MOSFET.
Power MOSFETs are typically arranged in an array of hundreds or thousands of individual MOSFET cells electrically connected in parallel. FIG. 1 shows a conventional single re-channel MOSFET cell 10 formed over p-type substrate 12. MOSFET cell 10 includes an N+ drain region 14, N+ source region 16, polysilicon gate structure 18, and n-channel 20. The insulating sidewall spacers 24 are formed around gate structure 18. Lightly doped drain (LDD) regions 26 are formed under sidewall spacers 24 to extend the horizontal conduction through n-channel 20 to drain region 14 and source region 16. A P+ source tie 28 can be formed by ion implantation.
N+ drain region 14 is coupled to a first operating potential, i.e., VDD. In typical operating conditions, VDD is 5-12 volts DC. N+ source region 16 is coupled to a second operating potential, i.e., ground potential. A voltage VG applied to gate structure 18 induces an electric field over re-channel 20 to cause a current to flow through drain region 14 and source region 16. MOSFET cell 10 has an inherent drain-source resistance (RDSON) in the conducting state. MOSFET cell 10 has a length L and width W and cell pitch of 1.36 micrometers (μm). The width W of MOSFET cell 10 controls the electrical resistance of the MOSFET cell. The larger the width W, the smaller the resistance. Typical values for width W are in the tens or hundreds of micrometers (μm). Each MOSFET cell 10 can be scaled to handle microamperes (pa) or perhaps milliamperes (ma) of drain-source current.
Many applications such as portable electronic devices and data processing centers require a low operating voltage, e.g., less than 5 VDC. The low voltage electronic equipment in the portable electronic devices and data processing centers create a demand for power supplies that can deliver the requisite operating potential.